1. Field of the Invention
The present invention relates to deep discharge protection for battery packs with cell groups, in particular for battery-powered hand-held power tools.
2. Description of the Prior Art
According to European Publication EP743736, a battery-powered hand-held power tool has a monitoring circuit that is integrated in the motor control for monitoring the charge state of the battery pack and which switches off the battery pack and/or the battery-powered hand-held power tool when a predetermined threshold value is not reached.
In a deep discharge of a battery pack, the cell with the smallest capacity reverses its polarity. Accordingly, an individual deep discharge of one cell can destroy an entire battery pack. In order to prevent this, every individual cell can be checked for a deep discharge protection by a voltage monitoring device to determine whether the voltage has fallen below a threshold voltage. However, this would be uneconomical in battery packs having many cells because of the high wiring costs. Therefore, usually the absolute group voltage over a plurality of cell groups is checked so as to reduce expenditure on wiring to an economically reasonable, smaller quantity of cell groups. However, individual cells cannot be checked directly in this way, so that the reversal of polarity of individual cells is difficult to prevent.
Further, according to U.S. Pat. No. 6,020,743, the change in the voltage difference over time between two series-connected cell groups having an identical quantity of cells is determined in a battery pack during the charging process. A charging process based on a charging of the battery pack that increases over time is unsuitable for a deep discharge protection during the discharge process with charging by a consumer that decreases over time.